The Worst Flu Seasons Since 2000
Every flu season ranked by peak severity — how high ILI% climbed, when it peaked, and how it compares to the most recent season. Data pulls live from CDC FluView ILINet via Delphi CMU.
| # | Season | Severity bar | Peak ILI% | Level | Peaked | Weeks active |
|---|---|---|---|---|---|---|
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Source: CDC FluView ILINet via Delphi CMU Epidata · wILI% = weighted influenza-like illness percentage of outpatient visits · Active weeks = weeks at Low (≥2%) or above · 2020–21 excluded from ranking (COVID precautions nearly eliminated flu that year — not a meaningful data point for severity comparison)
How severity is measured
The CDC measures flu activity through ILINet — a network of outpatient providers that report the percentage of visits for influenza-like illness (ILI) each week. The weighted percentage (wILI%) accounts for provider distribution across states and is the most reliable national indicator.
Peak wILI% is the best single number for comparing season severity. A season that hits 8% means roughly 1 in 12 outpatient visits that week were for flu-like symptoms — an extraordinary burden. A season at 3% is noticeable but not unusual. The worst seasons since 2000 tend to be dominated by H3N2 strains, which mutate rapidly and evade both natural immunity and vaccine protection more effectively than H1N1.
The 2009 pandemic is a special case: it produced two distinct waves (fall 2009 and winter 2010) and is reported across two flu seasons in ILINet. Its peak ILI% was moderate by severity standards, but its attack rate — the fraction of the population infected — was extraordinary due to zero prior immunity. The ILINet ranking understates pandemic seasons for this reason.